Modern railroad car wheel bearings are permanently lubricated sealed units designed to last for the life of the car. Sometimes, however, these wheel bearings fail during use, causing excess friction between the axle and the bearing and producing excess heat, resulting in a condition referred to as a "hot box". Normally, the bearings operate at about 20.degree. C. above the ambient temperature. When a bearing begins running at more than about 70.degree.-80.degree. C. above the ambient temperature, it has already failed. If the car continues moving at the same speed, internal fracture of a roller bearing can occur, causing the bearing to seize, and resulting in a thermal run-away. Under these conditions, the bearing temperature rises dramatically from about 20.degree. C. above ambient to more than 300.degree. C. above ambient temperature in about one-half mile of travel. Further travel melts the bearings which fall off the axle with the wheels causing the truck to fall to the ground, uncoupling the car from those in front of it and triggering the emergency brakes on the whole train. This causes the portion of the train behind the disabled car to collapse into an accordion-patterned wreck as railroad cars leave the track.
Brakes that fail to release also produce a dangerous condition that can cause a similar disaster. The affected wheel rises to temperatures on the order of 600.degree. C. and creates a condition known as a "hot wheel". If unchecked, the wheel ultimately disintegrates and a derailment results.
Because the hot box and the hot wheel can be so dangerous, the railroad service industry has devoted significant resources to building detectors that automatically check passing trains for hot boxes and/or hot wheels. Such detectors are conventionally spaced along railroad tracks at about twenty to fifty mile intervals along main-line track throughout the United States, and many are necessarily located in remote places. Although previous efforts have produced several sound products, a number of important problems have not been solved in the prior art.
Detectors in present use, typically include a sensing unit lens for focusing infrared from passing wheels onto an infrared sensor and electrical circuitry to develop a signal that is representative of the journal bearing or wheel temperature. One sensing unit is placed along one rail of the tracks and a second sensing unit is placed along the other rail of a set of tracks, so that both sides of a train can be monitored. Electrical lines connect these track-side sensing units to processing circuitry which is conventionally located in a "bungalow" close to the tracks. The final output signal of the detector can be used to create a written record of the temperature of each of the journals or wheels that passes the sensing units. In hot box detectors, this signal triggers a warning output if the signal indicates that the temperature of a wheel journal exceeds a predetermined value (generally about 70.degree.-80.degree. C. above ambient temperature).
While it is extremely important that no overheated bearings be missed by the hot box detector, it is almost equally important that no false alarms be generated since the unscheduled stopping of a train is a costly and time consuming operation that could result in substantial disruptions of schedules. The infra-red scanner and associated circuits for detecting overheated bearings are highly developed and available commercially from such sources as the Servo Corporation of America of Hicksville, N.Y.
Railroad freight cars in the United States usually have one of two types of bearings, plain bearings or roller bearings. Although plain bearings account for only approximately 16% of the effective rolling stock in the United States, the problems associated with the accurate analysis of signals from plain bearings are of particular importance to the railroad industry since in 1980, plain bearings accounted for 74% of the derailments. Because of different operating characteristics of the different types of bearings, the waveform of the infra-red scanner signal must be analyzed to permit proper bearing identification and proper alarm criteria must be set depending on the type of bearing imaged.
For purposes of the present discussion, the principal difference between roller and plain bearings which leads to problems in scanner signal analysis is that the portion of the plain bearing exposed to the infra-red scanner imaging spot is contained within a housing whereas the roller bearing is viewed directly by the scanner. The plain bearing housing, which protrudes from the car truck frame and is affixed to the frame, serves to siphon off some of the temperature rise of an operating bearing and dissipate it through the truck frame. Since a pair of bearing housings are usually provided on each truck frame located toward the ends of the frame, the surfaces of the housing facing each other (i.e., the housing inner sidewall surfaces) dissipate more heat than the surfaces of the housing facing away from each other (i.e., the housing outer sidewall surfaces). Since it is these inner and outer housing sidewall surfaces which are images by the hot box detector scanner, resultant signals from the scanner depend on which surface of the housing is imaged.
In addition to the above, since the housing protrudes from the truck frame, the leading surface of the housing is exposed to the cooling effect of the air stream generated by virtue of the train movement while the lagging surface is minimally effected by the air stream. The leading and lagging surfaces may be the inner or outer housing sidewall surface depending on the direction of the movement of the train. Roller bearings are not subjected to these problems since roller bearings are directly imaged and they are in rotation during imaging. As a result of the above, the accurate early detection of overheated plain bearings has been extremely difficult.
The prior art also includes the commonly used bolometer type of hot box and hot wheel detector. It employs temperature sensitive resistors (thermistors) in a bridge arrangement. Such units also require a highly stable and accurate high voltage supply. Because the signal-to-noise ratio of the bolometer decreases to unacceptable levels even within the normal operating temperature ranges of the detectors, automatic heaters must be installed to keep the thermistors warm enough to work properly. Once heaters are installed, it may become necessary to upgrade the optical system of the bolometer. Thus overcoming the fundamental problems inherent in a bolometer greatly complicates the device, making it more expensive to build and maintain, and less reliable. In addition, the frequency response of the bolometer is narrower than desired, restricting the top speed a train may be traveling while the bolometer checks for hot boxes or wheels.
Pyroelectric cells are also used as the infrared detection element in hot box and hot wheel detectors. Pyroelectric crystals acquire opposite electrical charges on opposite faces when subjected to a change in temperature. Pyroelectrical cells also exhibit some piezoelectrical properties, but the incidence of spurious signals generated by vibration have been virtually eliminated through physically isolating the cell from vibration. Pyroelectrical cells overcome many of the difficulties associated with bolometers. For example, hot box detectors built around pyroelectric detection schemes cost only about one-fifth to one-half as much as bolometers. Because the pyroelectric cell generates its own electric charge, large power supplies are not needed and the high impedance obviates the careful impedance matching of the bolometer. Further, no heaters are required because the signal-to-noise ratio is substantially flat over the required temperature range. Accordingly, simpler and cheaper optical systems can be used. Nevertheless, use of pyroelectric cells confronts the designer with other serious difficulties.
For example, pyroelectric cells tend to have an extremely poor voltage gain response when considered over any reasonable range of signal input frequencies, that is, over a range of train speeds. The voltage gain response tends to depend on the length of the time that the pyroelectric cell is exposed to the infrared, as well as the strength of the infrared. Thus, a typical infrared sensor employing a pyroelectric cell has an acceptably flat or constant voltage gain response over only about two percent of the frequency range required for acceptable hot detector operation, which is about 0.5 Hz to about 300 Hz. This prevents accurate temperature readings when a linear amplifier is used, yet only the voltage gain has a sufficiently high signal-to-noise ratio to provide a usable signal.
One prior art approach to overcoming this difficulty is to add a compensating signal to the pyroelectric cell signal to produce a signal having a flat frequency response over the normal range of frequencies, as set forth in U.S. Pat. No. 4,068,811. Over time, however, the breakpoint at which the voltage response of the pyroelectric cell begins to decline sharply drifts unpredictably due to changes in capacitance and response time. It may drift up or down the frequency scale; it may drift by different amounts. Neither the magnitude nor the direction of the drift will be the same for different detectors. The circuitry that develops the compensating signal cannot compensate for this drift, and so the detector will not produce the flat voltage response over the relevant frequency range that the remaining circuitry must have for proper operation. This long term signal drift requires frequent calibration checks of the pyroelectric cell. Such checks, and if necessary, re-calibration, are extremely difficult to perform accurately in the field and often require taking the unit to the shop. Even with frequent servicing, such units are often out of calibration and the resulting calibration errors lead to further reporting errors and increased service costs.
Another difficulty is created by the physical characteristics of pyroelectric crystals, namely that they produce an electrical potential only in response to changes in temperature. This characteristic requires that the infrared detector, that is, the pyroelectric cell, be subjected to changes in the amount of infrared striking it. In addition, the normal operating temperature of a railroad wheel bearing is determined relative to the ambient temperature. The requirement of measuring both the wheel bearing temperature and the ambient temperature provides a ready made opportunity to expose the pyroelectric cell to the required changes in infrared. Difficulties arise, however, in choosing a suitable infrared source to determine the ambient temperature.
Some pyroelectric hot box detectors in the prior art approach this problem by merely leaving the detector turned on whenever a train is passing and aiming the lens so that it receives infrared from passing bearings, and from the undercarriage of the railroad cars. This passive-read system assumes that the temperature reading developed from looking at the undercarriage is the ambient temperature, but if, for example, the undercarriage is on fire (which not infrequently occurs from faulty brakes), such a detector will see the heat from the fire as the ambient temperature and will be unable to detect any problem with a bearing, or even to detect the fire itself. Less dramatically, the sensor may measure the heat from a spurious source, such as brakes, and unable to distinguish between hot brakes and hot bearings, issue a hot box warning. Then the crew must stop the train, and walk the train searching for a non-existent over-heated bearing.
Another problem for passive-read systems is presented by the increased use of railroad spine cars, which are a skeleton steel-rail flatbed with trucks attached. Spine cars are used to haul semi-trailers piggyback. When a passive-read hot box detector looks at the undercarriage of spine cars, it is likely to take a "sky shot", and read only infrared from the distant sky as ambient. A sky shot temperature reading is usually about 20.degree. C. to 30.degree. C. less than actual ambient temperature. Naturally, this leads to many false warnings, since a bearing at normal operating temperature would show up as 40.degree. C. to 50.degree. C. hotter than the ambient temperature. Again, the crew must stop the train and walk the train searching for a non-existent hot box.
One prior art approach to overcoming this difficulty is to include a shutter that covers the lens at all times except when the apparatus expects to see a wheel bearing. This practice screens out all spurious infrared from overheated brakes and the like, and takes for its ambient temperature reading, the temperature of the shutter blade inside the detector housing. The detector, however, warms up and cools down more slowly than the true ambient temperature, especially during periods of rapid ambient temperature changes. These changes predictably occur around sunrise and sunset, and unpredictably occur during weather changes and in magnitudes that depend on the season and the weather. The temperature inside the detector housing tends to lag the actual ambient temperature by about two hours. This temperature lag can cause the measured difference between the correct ambient temperature and the journal bearing temperature to be wrong by as much as 10.degree. C. In addition, sun loading can heat the detector unit to a temperature that is considerably hotter than the ambient temperature. These differences between internal detector temperature and the actual ambient temperature can obviously lead to erroneous comparisons between ambient temperature and bearing temperature, creating both false negatives and false positives.
In addition, the prior art shutter detection scheme requires synchronization between the opening and closing of the shutter and the passing of the bearings, which necessitates rapidly starting and stopping the shutter. The shutter is operated by an electric solenoid. The ancillary devices required to synchronize the movement of the shutter with the passing train wheels are complex and expensive. Repeatedly energizing the shutter solenoid wears out the solenoid quickly, and the jolt caused by stopping the shutter sometimes creates spurious signals from the pyroelectric cell due to its piezoelectric characteristics. Accordingly, although use of a synchronized shutter to screen unwanted infrared from the pyroelectric cell avoids the temperature sensing problems of the passive-read system, it leads to complex problems of its own.
Therefore, a need exists for hot box and hot wheel detectors that are less expensive to manufacture, maintain, and operate; that are more reliable; that reduce or eliminate false negative and false positive warnings, both or which are inordinately expensive; and that produce consistent operating results over time by eliminating the effect of pyroelectric cell drift. In particular, a need exists for a direct temperature measuring device of the hot box itself, without the need for comparisons to ambient temperature measurements, which can upon being electromagnetically interrogated, detail the location of the failure, along with additional information pertinent to the failed or failing equipment.